[Patch]New InstCombine pattern for Icmp

[Yi Jiang]

With the help of Nuno and the Alive tool, I figured out the new condition.  The original condition actually refers to the following two ranges:
(icmp ult/ule (A + C1), C3) | (icmp ult/ule (A + C2), C3) ==  [MAX_UINT-C1+1, MAX_UINT-C1+1+C3] and [MAX_UINT-C2+1, MAX_UINT-C2+1+C3]
 We can fold these two range sif:
 1) C1 and C2 is unsigned greater than C3
 2) The two ranges are separated.  abs(LowRange1-LowRange2) > C3
 3) C1 ^ C2 is one-bit mask.
 4) LowRange1 ^ LowRange2 and HighRange1 ^ HighRange2 are one-bit mask.

Here is the new patch:


On Jul 30, 2014, at 2:24 PM, Nuno Lopes <nuno.lopes at ist.utl.pt> wrote:

> Hi Yi,
> 
> ALIVe still complains, I'm afraid:
> 
> Precondition: isPowerOf2(C1 ^ C2) && ugt(C1, C3) && ugt(C2, C3)
> %x = add %A, C1
> %i = icmp ult %x, C3
> %y = add %A, C2
> %j = icmp ult %y, C3
> %r = or %i, %j
> =>
> %and = and %A, ~(C1 ^ C2)
> %lhs = add %and, umax(C1, C2)
> %r = icmp ult %lhs, C3
> 
> Done: 1
> ERROR: Mismatch in values of i1 %r
> 
> Example:
> %A i2 = 2 (0x2)
> C1 i2 = 3 (0x3)
> %x i2 = 1 (0x1)
> C3 i2 = 1 (0x1)
> %i i1 = 0 (0x0)
> C2 i2 = 2 (0x2)
> %y i2 = 0 (0x0)
> %j i1 = 1 (0x1)
> %and i2 = 2 (0x2)
> %lhs i2 = 1 (0x1)
> Source value: 1 (0x1)
> Target value: 0 (0x0)
> 
> 
> Nuno
> 
> ----- Original Message ----- From: "Yi Jiang" <yjiang at apple.com>
> To: "Nuno Lopes" <nuno.lopes at ist.utl.pt>
> Cc: "LLVM Commits" <llvm-commits at cs.uiuc.edu>
> Sent: Wednesday, July 30, 2014 8:28 PM
> Subject: Re: [Patch]New InstCombine pattern for Icmp
> 
> 
> Hi Nuno,
> 
> Thank you for pointing out this! One condition is missing: C1 and C2 should be unsigned greater than C3.
> Here is the new patch.
> 
> -Yi
> 
> On Jul 29, 2014, at 11:34 AM, Nuno Lopes <nuno.lopes at ist.utl.pt> wrote:
> 
>> Hi,
>> 
>> I sent you an email the other day, but it was rejected by Apple's email server.  Tyring now through another email account.
>> Please take a look here: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140721/227575.html
>> 
>> Nuno
>> 
>> ----- Original Message -----
>> 
>> Ping...
>> On Jul 25, 2014, at 4:11 PM, Yi Jiang <yjiang at apple.com> wrote:
>> 
>>> Hi Ben,
>>> 
>>> Thank you for your comments! Here is a new version.  In this version, I still kept some "if" thinking that some operand has been already extracted so I would like to use it directly.  Please let me know if any other comments.
>>> <combinecmpV2.patch>
>>> -Yi
>>> On Jul 25, 2014, at 2:12 AM, Benjamin Kramer <benny.kra at gmail.com> wrote:
>>> 
>>>> On Fri, Jul 25, 2014 at 1:24 AM, Yi Jiang <yjiang at apple.com> wrote:
>>>>> Hi,
>>>>> 
>>>>> his patch is trying to fold (icmp ult/ule (A + C1), C3) | (icmp ult/ule (A +
>>>>> C2), C3)  to (icmp ult/ule ((A & ~(C1 ^ C2)) + max(C1, C2)), C3)  .
>>>>> This transformation is legal if C1 ^ C2 is one-bit mask, in other word, C1
>>>>> is only one bit different from C2. In this case, we can "mask" that bit and
>>>>> do just one comparison.
>>>>> A typical example is:
>>>>> isALPHA(c)    ((c) >= 'A' && (c) <= 'Z') || ((c) >= 'a' && (c) <= 'z')
>>>>> 
>>>>> Now llvm will optimize it to ((c + 191) <=25) || ((c + 159) <=25)
>>>>> 
>>>>> With this patch, we can optimize it further to:
>>>>> (c & 223) + 191 <= 25
>>>>> 
>>>>> The binary format of the constants are:
>>>>> 191   10111111
>>>>> 159   10011111
>>>>> 223   11011111
>>>>> 
>>>>> Here is some experiment result on arm64:
>>>>> The patch shows no regression and improve spec2000 perlbmk 3.8% in
>>>>> test-suite under -O3.
>>>>> We also test the spec2006 400.perlbench with ref size input, it will improve
>>>>> 1% under -O3 and 1.2% under -O3+lto+pgo.
>>>> 
>>>> Wow, very nice.
>>>> 
>>>>> Any comments are appreciated.
>>>> 
>>>> The patch could be significantly simplified with the PatternMatch
>>>> tool. Something like match(LHS, m_Add(m_OneUse(m_Value(A),
>>>> m_ConstantInt(LAddCst)... could replace your if chain.
>>>> 
>>>> - Ben 
> 

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